In biotin-dependent carboxylases, the biotin moiety is covalently bonded to the Biotin Carboxyl Carrier (BCC) functionality of the enzyme via an amide linkage to the epsilon amino group of a single lysine residue. In E. coli acetyl-CoA carboxylase, the BCC functionality resides in a separate 17 kD subunit. An 87 residue C-terminal domain fragment (BCCP87) of this subunit has been shown to behave identically to the intact protein in the enzyme-catalyzed biotination reaction. High resolution structures of both the apo- and holo-forms of the domain are available. The folding properties of the apo- and holo-forms of BCCP87 will be studied by differential scanning calorimetry to determine the contribution, if any, of a post-translational modification to the thermodynamic stability of the protein. Results of the thermodynamic studies will be combined with the available structural data to determine the structural basis of any measured differences in the energetics of folding.